Re: [PATCH] mm: vmalloc: Remove double execution of vunmap_page_range

[Chintan Pandya]

On 4/13/2018 4:39 PM, Michal Hocko wrote:
> On Fri 13-04-18 16:15:26, Chintan Pandya wrote:
> > On 4/13/2018 4:10 PM, Anshuman Khandual wrote:
> > > On 04/13/2018 03:47 PM, Chintan Pandya wrote:
> > > > On 4/13/2018 3:29 PM, Anshuman Khandual wrote:
> > > > > On 04/13/2018 02:46 PM, Chintan Pandya wrote:
> > > > > > Unmap legs do call vunmap_page_range() irrespective of
> > > > > > debug_pagealloc_enabled() is enabled or not. So, remove
> > > > > > redundant check and optional vunmap_page_range() routines.
> > > > >
> > > > > vunmap_page_range() tears down the page table entries and does
> > > > > not really flush related TLB entries normally unless page alloc
> > > > > debug is enabled where it wants to make sure no stale mapping is
> > > > > still around for debug purpose. Deferring TLB flush improves
> > > > > performance. This patch will force TLB flush during each page
> > > > > table tear down and hence not desirable.
> > > > Deferred TLB invalidation will surely improve performance. But force
> > > > flush can help in detecting invalid access right then and there. I
> > >
> > > Deferred TLB invalidation was a choice made some time ago with the
> > > commit db64fe02258f1507e ("mm: rewrite vmap layer") as these vmalloc
> > > mappings wont be used other than inside the kernel and TLB gets
> > > flushed when they are reused. This way it can still avail the benefit
> > > of deferred TLB flushing without exposing itself to invalid accesses.
> > >
> > > > chose later. May be I should have clean up the vmap tear down code
> > > > as well where it actually does the TLB invalidation.
> > > >
> > > > Or make TLB invalidation in free_unmap_vmap_area() be dependent upon
> > > > debug_pagealloc_enabled().
> > >
> > > Immediate TLB invalidation needs to be dependent on debug_pagealloc_
> > > enabled() and should be done only for debug purpose. Contrary to that
> > > is not desirable.
> > Okay. I will raise v2 for that.
>
> More importantly. Your changelog absolutely lacks the _why_ part. It
> just states what the code does which is not all that hard to read from
> the diff. It is usually much more important to present _why_ the patch
> is an improvement and worth merging.

It is improving performance in debug scenario. More than that, I see it
as a clean up. Sure, I will try to address *why* in next change log.

Chintan
--
Qualcomm India Private Limited, on behalf of Qualcomm Innovation Center,
Inc. is a member of the Code Aurora Forum, a Linux Foundation
Collaborative Project